Lecture 8: Cystic Fibrosis: pathogenesis & patient management Flashcards
Describe the Overview of the CFTR protein function
• Single polypeptide chain, 1480 amino acids with array of normal functions:
– Volume (liquid ~water) absorbing (airway, distal intestine)
– Salt absorbing without volume (sweat ducts)
– Volume secretory (proximal intestine, pancreas)
Explain the Structure and function of CFTR protein
• Consists of five domains:
- Two (trans) membrane-spanning domains (TMD1 and TMD2) =form the chloride ion channel across the membrane + anchor into membrane
- Two nucleotide-binding domains (NBD1 and NBD2) = bind and hydrolyse ATP
stimulate channel opening and closing
- Channel regulated by (de)phosphorylation by kinase- A-> alters interactions of the regulatory (R) domain
Phosphorylation= open channel
Dephosphorylation= close channel
- Clues to open and close channel
-> come from NBDs and R-> via intracellular loops (ICLs) that protrude from TMDs into cytoplasm
Explain the mutations of CFTR protein
- Delta F508 (most common CF-causing mutation) -> codes for first nucleotide-binding domain (NBD1)
- Deletion of F508 in NBD1 F508 -> leads to improper processing + folding + intracellular degradation of the CFTR protein by ER
- Domain-assembly defects can occur -> F508 interacts with ICL4 in TMD2
= creating a cavity + disrupting the NBD1–TMD2 interface
critical for folding and function - Other mutations in the CF gene produce fully processed CFTR proteins that are either non-functional or partially functional
- Mutations can be identified by sequencing, assigned to a protein residue and a functional domain if possible
- The functional significance of a mutation can be assessed experimentally
- Possible methods of reverting effect of the mutation can be determined
Explain the effects of airway disease due to CFTR mutation
- Loss of normal CFTR function and over activity of ENaC (epithelial Na channel) cause:
1. Loss of HCO3− secretion (acidification)
2. Airway surface liquid layer (ASL) dehydration disrupts mucociliary escalator
3. ASL viscosity, mucus plugging, bacterial colonization, and neutrophil-dominated inflammation
4. Bacterial and neutrophil derived proteases degrade antiproteases and antimicrobial peptides in the CF airways
5. Compromised innate immune system
List and define the different classes of CFTR mutants
- class 1= premature termination of CFTR mRNA translation
- class 2= degradation in the ER
- class 3= regulatory mutants that fail to respond normally to activation function- protein present but non/partial functional
- class 4= CTFR mutants that have altered channel properties
- class 5= decreased functional CFTR synthesis or transport
What do you have to consider about the classes when it comes to treatment?
Have to consider different treatments for classic and non-classic CF-> target different abnormalities that are less severe for some
Describe the features of classical and non-classical CF
Classic= non-functional CFTR
- chronic sinusitis
- severe chronic bacterial infection in airways
- severe hepatobiliary disease
- pancreatic exocrine insufficiency
- meconium ileus at birth
- sweat chloride value usually between 90-110 mmol/L or 60-90 mmol/L
- obstructive azoospermia
Nonclassical= some fucntion of CFTR-> provide survival advantage
- chronic sinusitis
- chronic bacterial infection of airways-> later onset , variable
- adequate pancreatic exocrine function -> pancreatitis
- sweat chloride value between 60-90 mmol/L, sometimes < 40 mmol/L
- obstructive azoospermia
Explain how genotype can correlate to phenotypes
- Variation of genotype-> provides reason for phenotypic effects of specific mutation
Explain how the variation of severity of CF can occur
- Mild allele-> different mutation -> heterozygosity= 2 different CF mutations from each person come together to form mild effect= non-classic
- Homozygous for 1 mutation-> strong effect= classic
- Classic alleles-> Class I-III mutations
- Mild alleles-> Class IV-V
Give a mutation and describe each clinical phenotype of CF and
- least affected= milder
- miler lung disease
- pancreatic insufficiency
- abnormal sweat chloride
R117H (5T) - Mild
- mild lung disease
- pancreatic sufficiency
- equivocal sweat chloride
- R117H (7T)
- Severe
- severe lung disease
- pancreatic insufficiency
- abnormal sweat chloride
- delta F508
Explain how the genotype - phenotype correlation for CF can be modified
- Limited correlation between CFTR genotype +severity of lung disease
= suggests environmental and genetic modifiers - Several candidate genes related to innate and adaptive immune response- potential pulmonary CF modifiers
- A genetic CF modifier for meconium ileus = human chromosome 19q13.2
- Phenotypic spectrum associated with CFTR mutations ranges classically defined CF
= Patients with atypical CF - Monosymptomatic diseases: obstructive azoospermia, idiopathic pancreatitis or disseminated bronchiectasis
- > associated with CFTR mutations + is uncharacteristic for CF
- Other gene can interfere with CF phenotype -> affect susceptibility to certain bacterial infection
Explain the challenges of treatment identified by pathophysiology
Lung
- High rate of sodium absorption + low rate of chloride secretion
=reduces salt and water content in mucus -> decrease in liquid surrounding cilia
- Mucus adheres to airway surface
= decreased mucus clearing
->predisposition to Staph and Pseudomonas infections
Gastrointestinal
• Pancreas
- Absence of CFTR-> limits function of chloride-bicarbonate exchanger
=Retention of pancreatic enzymes in pancreas
= Autolysis of pancreas- destruction of pancreatic tissue
• Intestine
- Decrease in water secretion
=leads to thickened mucus
= desiccated intraluminal contents and obstruction of small and large intestines
• Biliary tree
- Retention of biliary secretion and focal biliary cirrhosis (late stage of scarring (fibrosis) of the liver)
= Bile duct proliferation, chronic cholecystitis, cholelithiasis
